1. Field of the Invention
The present invention relates generally to gamma ray detection and more particularly to semiconductor gamma ray detectors.
2. Description of the Related Art
Gamma rays have wavelengths which are shorter than 10.sup.-10 meters and energies (E.gamma.) which are inversely proportional to their wavelengths. When they interact with a semiconductor that has an energy bandgap Eg, approximately E.gamma./3Eg electron-hole pairs are generated (the number 3 in this equation is an efficiency factor). For example, the bandgap energy of cadmium telluride (CdTe) is approximately 1.5 eV. Thus, a gamma ray with E.gamma.=150keV will generate 150.times.10.sup.3 /4.5=33.3.times.10.sup.3 electron-hole pairs.
CdTe and cadmium zinc telluride (CdZnTe) have proven to be particularly useful in the art of gamma ray detection. Each of these compound semiconductors has a wide energy bandgap and can be grown by processes, e.g., traveling heater method (THM) and the Bridgman method, which yield high-resistivity crystals (for further growth details, see Sen, S., et al. "Crystal Growth of Large-Area Single-Crystal CdTe and CdZnTe", Journal of Crystal Growth 86, 1988, North-Holland, Amsterdam, pp. 111-117). These two parameters, wide bandgap and high resistivity, inhibit noise-producing leakage currents. In addition, because Cd and Te have relatively high atomic numbers (48 and 50), they present a large number of electrons for interaction with incident gamma rays. This enhances electron-hole generation and facilitates the production of detectable current signals. Finally, the production technology of these semiconductors has matured to the point that products designed to include them are commercially viable. Zn is typically added to CdTe to further widen the bandgap. However, the percentage of Zn is usually limited because of its low atomic number.
CdTe and CdZnTe are respectively binary and ternary compound semiconductors that are formed from elements of the II and VI columns of the periodic table. Accordingly, they are typically referred to as II-VI semiconductors. A ternary semiconductor is sometimes called an alloy semiconductor because one sublattice of the semiconductor crystal is shared in an alloy composition. The alloy composition of CdZnTe is typically written as Cd.sub.1-x Zn.sub.x Te which means that Zn atoms and Cd atoms are randomly mixed in the crystal's group II sublattice in the ratio of x and 1-x mole fractions. In this composition, all sites on the group VI sublattice are occupied by Te atoms.
This style of nomenclature can be easily extended to quaternary compounds. For example, In.sub.x Ga.sub.1-x-y Al.sub.y As describes a III-V compound in which In atoms, Ga atoms and Al atoms are randomly mixed in the crystal's group III sublattice in the ratio of x, y and 1-x-y mole fractions. It is customary to allocate the x or y designation to the element or elements that have a minority alloy percentage.
Detectors built with CdTe and CdZnTe are used to form gamma ray counters and gamma ray spectrometers. A gamma ray counter provides a simple count of incident gamma rays. In addition, a gamma ray spectrometer distinguishes between energy levels of incident gamma rays so that the population of gamma rays at each incident energy level (or each incident wavelength) is determined. Gamma ray spectrometers are used in a variety of technical fields, e.g., nuclear medicine, nuclear fuel characterization, satellite astronomy and chemical analysis in oil well bores. For further background discussion of CdTe detectors and for models of energy levels that are associated with crystal defects, see Siffert, P., "Cadmium Telluride Detectors and Applications", Material Resource Symposium Proceedings, Vol. 16, 1983, pp. 87-114).
In a gamma ray spectrometer, an electrical bias is typically applied to sweep charge carriers (electrons and holes) from the detector to form a detectable current pulse. The amplitude of the pulse is proportional to the number of generated electron-hole pairs (i.e., inversely proportional to the radiation wavelength) and the duration of the pulse is a function of the electron and hole collection times.
In practice, a number of factors decrease the amplitude and increase the time duration of the detected pulses. These factors cause a spectrometer's ability to distinguish between gamma ray wavelengths (i.e., its resolution) to be degraded. Among the most important factors are native and foreign defects in the semiconductor crystal of the gamma ray detector.
Accordingly, methods have been developed for reducing the number of foreign defects, i.e., impurity content, in CdTe and CdZnTe. One approach is to reduce impurities in the starting materials of elemental Cd and Te, e.g., by the processes of distillation and zone refining. Another approach is to reduce the impurity content of grown CdTe and CdZnTe crystals, e.g., by surface damage gettering. CdTe wafers have also been annealed in the presence of Cd vapor with the objective of improving the crystalline quality by adjusting the crystal stoichiometry and reducing Te precipitates.
Methods for reducing gross and point native defects have generally centered on the control of crystal growth conditions. For example, temperature, temperature ramp rates, vapor overpressure and growth direction can be carefully manipulated to limit gross crystal defects, e.g., grain boundaries and twin boundaries. Typically, a lower priority has been placed on the reduction of native point defects, e.g., vacancies (a missing atom at a crystal lattice site) and interstitials (an atom positioned between lattice sites).